For the power supply of light sources such as halogen lamps, for example halogen lamps with low supply voltage, electronic transformers are used which can perform an intensity regulating function on the light source, i.e. a so-called “dimming” function.
The implementation of this function makes use of the fact that the light intensity of such sources is dependent on the (average) intensity of the current flowing through the light source.
The intensity regulating device—named “dimmer” for brevity—operates so to say by “cutting” the sine wave form, which normally has already undergone a rectification, via an operation of phase cut.
This function can be performed both on the rising edge and on the falling edge of the sine (half) wave. The devices named “phase-cut dimmers” can therefore be classified into two groups:                devices acting on the rising edge or on the front of the sine (half) wave, i.e. at the beginning of the period at 100 Hz (reference is obviously being made to a sine wave form at 50 Hz, which has already undergone a half-wave rectification),        devices acting on the falling edge, or on the tail of the sine (half) wave at 100 Hz.        
The devices of the first kind, known as “leading edge dimmers” are the more widespread at present, because they are more economical to produce.
The electronic transformers currently employed in the presently considered applications normally include a self-oscillating half-bridge topology, adapted to work suitably with phase-cut dimmers of the previously considered type.
In the case of electronic transformers having a rather high power (for example an input power of 300 W), the use of a self-oscillating topology is more difficult. This is due to the need of a suitable control of input and output currents and of output voltages, particularly during start-up and in protection stages against abnormal operating conditions (overload, overheating, over-temperature).
In order to properly control the power stage, it is then possible to provide a processor, such as a digital microcontroller, combined with an external driver. Both the processor (microcontroller) and the driver require a constant voltage supply, usually of the order of a few Volts (Vcc). For reasons due to energy saving requirements (specifically in order to reduce consumption in a stand-by mode), this voltage is obtained with a Switch Mode Power Supply stage (SMPS).
FIG. 1 is a block diagram showing a solution corresponding to what has been previously described.
Specifically, in the block diagram of FIG. 1, reference 10 denotes a power stage including, for example, two electronic switches (for example power mosfets) adapted to be alternatively switched on and off, i.e. to be made conductive and non-conductive, associated with respective capacitors 14 in a self-oscillating half-bridge arrangement, adapted to drive the primary winding 16a of a transformer 16. The secondary winding 16b of transformer 16 feeds load L, which is a lamp or lamps (which of course, though shown in the drawing, are not in themselves a part of the supply circuit).
In the example considered in FIG. 1, feeding power stage 10 from mains M is achieved with an input filter 18 and a diode bridge rectifier 20, wherefrom a feed line 21 from mains branches which has a “bus” voltage Vbus, adapted to feed power stage 10.
Reference 22 denotes the drive stage or driver, which turns the switches 12 in power stage 10 on and off alternatively, on the basis of controls received from a processor such as microcontroller 24.
Reference 26 identifies a supply stage (Switch Mode Power Supply stage or SMPS) connected to the feed line 21 from mains. On the output of stage 26 a direct voltage Vcc is present which is adapted to be used as a supply voltage for driver 22 and for the microcontroller processor 24.
Finally, reference 28 denotes a phase-cut dimmer (which is assumed to be interposed between the input of mains voltage M and the input filter 18) which, by operating according to well-known criteria, performs a “cutting” function on the wave form of the mains supply; under the action of an external dimming control (produced according to well-known criteria and means), dimmer 28 is therefore selectively switchable between:                a conductive state (wherein the mains supply flows to the device) and        a non-conductive state (wherein the mains supply to the device is interrupted),        
so as to either permit or interrupt the supply to the device from mains.
The circuit topology shown in FIG. 1 is to be considered as known in itself, which makes it unnecessary to provide for its detailed description herein.
It will be realized, moreover, that in order to solve the technical problem explained in the following, the circuit arrangement of FIG. 1 must be considered exemplary, in general terms, of the topologies of the power supply device for light sources wherein the device includes:                a feed line from mains (e.g. line 21) through a phase-cut dimmer which performs a “cutting” function on the wave form of the supply from mains, the dimmer being selectively switchable between a conductive state (wherein the supply from mains flows to the device) and a non-conductive state (wherein the supply from mains to the devices is interrupted), so as to either permit or interrupt the supply from mains to the device,        a power stage to feed at least one light source from said feed line from mains,        a drive stage for the power stage, and        a supply stage for the drive stage, said supply stage being connected to said feed line from mains.        